1. Field of the Invention
The present invention relates to linear actuators.
2. The Prior Art
Linear actuators are for example known from EP 531 247 A1, EP 586 326 A1, EP 647 799 and EP 662 573 A1 all belonging to Linak A/S. The forces occurring on the spindle are absorbed by a push/pull bearing encapsulated in a plastic housing. The forces occurring between the bearing and the rear mounting are transferred through the plastic housing and the housing must be dimensioned and designed accordingly. Such a plastic housing constitutes a considerable amount of the actuators price. From WO 98/30816 Linak A/S a sophisticated actuator with a very high performance is disclosed where the forces between the bearing and the rear mounting are transferred via a metal chassis. These actuators are amongst others used in furniture, for example, in hospital and care beds, where is a requirement that the actuators are damp and waterproof. As an example, see EP 488 552 A1 Huntleigh Technology plc. As the actuators are completely or partly visible, it is additionally a requirement that the actuators have a minimum of design with smooth and dirt-resistant surfaces.
A designless heavy duty actuator without a housing for industrial purposes is based on a supporting metal housing with integrated rear mounting, where the forces are transferred through this.
An actuator of this type is disclosed in DE GM 94 04 383.3 U1, where the housing, which is of pressure die cast zinc, is formed with a cylindrical portion for the motor, the cylindrical part can be closed with a cover. In addition, the actuator is waterproof, with suitable sealing, such that it can be used for beds within the hospital and care sector. The pressure die cast zinc housing, which must be relatively large because it has to include the motor, makes it a relatively expensive solution.
From EP 0 831 250 A2 is known an actuator with a housing. The housing includes a shell, which surrounds the motor and gearing. The shell, which is open to the rear that is towards the rear mounting of the actuator, is closed with a cover. To the front that is to towards the spindle, the shell is shaped with a fixing for the outer tube, said outer tube acting as a guide for the spindle nut and as a guide for the outer end of the spindle. The transversal forces, which in any case occur on the spindle, are transferred via the outer tube to the housing. It is accordingly stated that the housing and the outer tube are made of a rigid material and as an example are amongst others specified aluminum, zinc alloys and stainless steel, that is to say the specified plastic material, glass fibers containing nylon, polyvinyl chloride and polyethylene have an equivalent rigidness. On the whole a relatively expensive solution.
The purpose of the invention is to provide an actuator of the type first mentioned, which is cheap to produce, but in terms of performance and quality is equal to these.